If its a definition you want, you can check this link: http://en.wikipedia.org/wiki/Thermal_runaway
However, i do not think what you need is a definition. Rather, what you need is a good understanding of the underlying principles described in the definition. You should do some reading on the ff:
1. The effect a change in temperature has on the resistance of a semiconductor, and by extension on a P-N junction diode.
2. The 'diode-equation' and how a variation in temp. affects the other parameters in that equation.
3. The I-V characteristics of a P-N junction, and how a change in temp. affects the characteristics (specifically, how the slope is affected).

Hi
i got question in my previous year question paper that Define thermal runway in power amplifier section but could not able to get myself satisfied with the answer that got from my book.

Will you please help to get a satisfactory answer.
from Sabbir

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thermal runaway is an increase in collector current due to a small increase in base current of a bjt transistor which leads to a Rc heat up.To overcome thermal runaway we connect an emitter resistor in parallel to Rc.